In typical receiver equipment, particularly radio receivers, imperfections or design compromises lead to unwanted spurious signals being introduced. A common type of spurious signal is zero frequency (DC) components in the signal, generated either directly, such as occurs due to e.g. bias voltages in amplifiers, or resulting spurious signals whose frequency is shifted by subsequent signal processing so that they lie at DC, as can occur in radio receiver systems using a sampled digital low-IF concept. Such signals generally interfere with the proper operation of digital receiver circuitry, and therefore must be removed by some form of compensation.
A typical receiver performs automatic gain control (AGC), which periodically adjusts the gain of the receiver to match the strength of the incoming signal. Since spurious signals generated within the analogue receiver will experience this changing gain, the resulting DC signal level will vary, making the task of eliminating the signal difficult. A simplified example is shown in FIG. 1 where the RF mixer 106 introduces a small DC offset that is subsequently amplified by the programmable PGA 102 gain, which PGA 102 is controlled by a digital controller 103, and where the DC offset appears on the input to the analogue-to-digital converter (ADC) 101 on the digital chip 105.
AGC is most often used while searching for the start of a signal. Once a signal has been detected, a fixed gain setting may be chosen to match the strength of the incoming signal. This detection process is often time-critical. For example, in the 802.11 wireless LAN standard there is a defined maximum time by which the presence of valid signals must be indicated. This means that any method for compensating for DC offsets must not delay or distort the signal so that more time is required to distinguish a valid signal.
A standard method for eliminating DC components in a signal is to use a DC-blocking digital notch filter. This has the properties of completely blocking DC signals while passing higher frequencies, with very small delay to frequencies significantly higher than the cut-off frequency.
A second standard method for eliminating DC components in a signal is to use DC estimation and subtraction, where the mean signal level is measured over a period of time to calculate an estimate of the DC level, after which the estimate is subtracted from the data samples to remove the DC component.
One problem with the notch filter approach is that it introduces phase and amplitude distortion to the signal. The amount of distortion can be minimised by making the filter cut-off frequency small compared to the lowest wanted signal frequency. However, the filter also has a finite response time to step changes in DC level, which is proportional to the inverse of the filter cut-off frequency. The result is that when the AGC process changes the signal gain, the resulting step change in DC level at the input to the filter causes a transient pulse at the output.
The problem with the estimation/subtraction method is that the accuracy of the DC estimation depends on the number of samples over which the estimate can be performed. This either introduces a delay in the signal, or requires that the samples contributing to the DC estimate are not correctly compensated.